1. Field of the Invention
The present invention relates to a zoom lens system and an image pickup device adopting the zoom lens system as an image pickup lens, and particularly to a compact zoom lens system and a small-sized image pickup device adopting the compact zoom lens system, such as digital still cameras and home video cameras.
2. Description of Prior Art
Recently, digital still cameras and digital video cameras have been widely used in household. With the increasingly compact design of an image sensor used in these image pickup devices, such as a CCD (Charge Coupled Device) sensor or a CMOS (Complementary Metal Oxide Semiconductor) sensor, the image pickup device adopting the image sensor also has been required to be further compact to meet the trend of small size, light weight and portability. Correspondingly, imaging lenses mounted in these image pickup devices, especially for zoom lens systems, also have been designed to be compact by shortening the overall length thereof. Further, for these imaging lenses, especially for those used in digital still cameras, besides the compact requirement, the lens performance is also required to be increased to accord with an image pickup device with high image resolution.
Conventionally, for a zoom lens system having a magnification or zoom ratio of about 3×, the configuration of three lens groups is widely adopted for the sake of shortening the overall length of the zoom lens system. This kind of zoom lens system is disclosed, for example, in U.S. Pat. No. 5,909,318. The '318 patent discloses a three-group telescopic zoom lens system which in use can be outstretched beyond the body of a camera accommodating it and each lens group of which is movable. When not in use, each lens group of this telescopic zoom lens system is retracted back into the camera body, so as to shorten the overall length. However, such a telescopic zoom lens system is quite limited in dust, water and drop protection since in use the lens barrel supporting the movable lens groups needs to be extended out of the camera body.
Accordingly, an inner focusing zoom lens system with fixed overall length has been proposed in the art to meet both compactness and durability requirements. Such an inner focusing zoom lens system is disclosed, for example, in U.S. Pat. Nos. 7,068,441; 7,242,529 and 7,256,945. The zoom lens systems disclosed by these patents each have a first lens group fixedly positioned for increasing the durability of the lens. In addition, a right-angle prism is arranged in the first lens group to reflect the incident light from the object by 90 degrees, so as to shorten the overall length of the optical system. However, each of the zoom lens systems disclosed by these patents is configured with five lens groups, most of which are movable to effect zooming and focusing. These movable lens groups are moved over a relatively long distance, thereby increasing the overall length of the zoom lens system. In addition, besides the right-angle prism, each zoom lens system is comprised of 9 to 11 lens elements. This further extends the overall length of the zoom lens system and thus increases the size, cost and weight of an image pickup device adopting this zoom lens system.
Hence, the above-mentioned conventional inner focusing zoom lens systems with a light path bending element still need to be improved to follow the compactness and low-cost trend of image pickup devices while providing comparable imaging performance.